1. Field of the Invention
The invention relates generally to the field of scaffolding, and in particular concerns improved structural designs of scaffold frame construction elements providing improved strength and load capacity in open-frame scaffold systems.
2. Prior Art
Scaffold systems are widespread in industrial, commercial and residential construction settings. Scaffolds generally involve elements that must be compact and light in weight to be carried while climbing, but that assemble into robust structures that vary in size and shape. Conventional scaffold systems are made from a plurality of modular support assemblies coupled by horizontal and diagonal cross-braces to create scaffold frames. Scaffold planks are secured on horizontal members of these frames and provide a platform on which workers and tools can be supported.
Scaffold systems typically are erected as freestanding structures along and beside objects upon which work is to be performed. Scaffold structures are also commonly used for shoring, i.e., supporting a weight. They must be rigid enough to safely support heavy loads including workers, equipment and other structures, yet be structurally simple in that assembly and disassembly of the structures are easily and quickly achieved.
Conventional scaffold systems use horizontal and diagonal cross-braces on several faces of the scaffolding frame to provide necessary structural rigidity and load bearing capacity. An example of a truss based design of this type is disclosed in U.S. Pat. No. 3,190,405--Squire, wherein a scaffold is made from support assemblies having two horizontally spaced vertical standards coupled by horizontal cross-braces welded between the standards. These support assemblies couple vertically with one another. Pairs of the support assemblies on each level and support assemblies on successive levels are connected by removable cross-bracing extending diagonally between the standards. Diagonal cross bracing is also connected between the corners of individual support assemblies for keeping them rectilinear against the tendency to collapse into the shape of a parallelogram. A horizontal deck is supported on the top of the standards, and one or more similar decks could be placed on the horizontal cross members at any of the levels.
While cross-bracing provides an effective means for stabilizing and strengthening a scaffold system, it presents impediments for contractors attempting to work. Horizontal and diagonal cross-braces, particularly on the side of the scaffold facing a structure, obstruct free access to the structure for workers. Similarly, cross-bracing on the opposite side obstruct access for climbing onto the decks and cross-bracing on individual support assemblies obstructs lateral walkways through the scaffold frame. Thus the cross-bracing, which is needed for structural rigidity, requires workers to maneuver themselves and their equipment awkwardly around cross-brace members when traversing the scaffold.
Cross-braced designs are more difficult to erect and dismantle than systems which are made rigid in a different manner. For example, heavier standards and horizontal members can support a load as well as lighter standards and horizontal members coupled with cross-bracing. Thus, the heavier members can permit better access and open walkways. Rectilinear support assemblies of sufficient strength can avoid the need to install removable cross-braces, a time consuming procedure, and improve efficiency by providing better access and freedom of movement. However, heavy support assemblies likewise present problems in that they must be carried up lower support assemblies and installed when building a scaffold having a plurality of levels.
Attempts have been made to reduce the number of cross-braces used in scaffold systems while maintaining adequate structural rigidity and load capacity. Referring to FIG. 9 herein, labelled as prior art, an example of a scaffold system 200 having a reduced number of cross-brace members is the Modular Frame Scaffold System sold under the tradename SPRINT. This design omits cross-bracing from the work-side face of the scaffold structure and from the scaffold thruways (i.e., through the support assemblies at the ends) to create what can best be characterized as an open-frame system. Short diagonal gussets 230 are provided in each of the upper corners of the modular support assembly, in lieu of obstructive diagonal cross members. A single diagonal brace 215 is coupled between the upper end of one vertical standard of one support assembly and the lower end of the next. Each gusset member is welded to a top horizontal cross member 232 and the adjacent support column 212, creating a moment connection which provides structural rigidity to the frame in a direction toward and away from the structure. The longer diagonal brace 215 in the embodiment shown provides rigidity as to only one of the two support assemblies; however, where more than two support assemblies are used together, further diagonal braces 215 (not shown) support the other support assembly as well.
The SPRINT system employs plank members 205 to transfer horizontal loads from the front or work-side leg of the scaffold to diagonal cross-braces 215 on the rear of the scaffold assembly. This is accomplished by providing a series of hooks 225 at each end of the planks which structurally connect with the top cross members of adjacent support assemblies to provide a rigid connection. Such load bearing planks preferably are installed at every level of the SPRINT scaffold system and compensate for the omitted front diagonal cross-bracing. Thus this open-frame design can be safely built to heights attained by conventional scaffold systems. However the scaffold elements, and in particular the support assemblies, are heavier than those of cross-braced scaffolds as in U.S. Pat. No. 3,190,405--Squire, cited above.
With further reference to FIG. 9, a lower horizontal cross member 110 is provided on the support assembly of the SPRINT scaffold and is welded between paired vertical standards or support columns 212. The lower horizontal member is made substantially from rectangular tubing with tapered ends, as detailed in FIGS. 6-8. Lower cross member 110 is formed from an elongated rectangular tube that is crushed down at the top such that its cross-section resembles a "T" shape having a relatively wide upper surface, part of which is two thicknesses, carried on the remainder of the rectangular tubing, which provides support from below (FIG. 7). The rectangular tubing of the lower cross member is also folded inwardly approaching the ends to create a tapered section at each end of the member (FIG. 8). The tapered section clears the extreme ends of the vertical columns, which protrude and can be swaged such that the lower ends of the vertical columns fit into the upper ends of a similar support assembly, or vice versa, when the scaffold frames are stacked to form plural levels. Alternatively, a connection pin with an enlarged center between oppositely protruding ends can be inserted into column ends of equal diameter that are abutted end to end. The lower cross member also prevents planks 205 from lifting up from the top cross member 232 at any intermediate level of decking, which would disengage hooks 225 from top cross member 232 and result in a loss of structural support.
The SPRINT design addresses the access disadvantages associated with conventional scaffolding systems having numerous removable diagonal braces, but it also introduces new limitations. The minimally braced frame design unavoidably permits more lateral sway in the plane of the frame than one with long diagonal bracing. In fact, the legs of such open frames are considered to behave structurally as if they are longer than they actually are, for example as much as 40% longer. This increased virtual length decreases load carrying capacity of the scaffold system as a practical matter because the load bearing ability of the column varies inversely to the square of its length or virtual length.
Due to the tapered ends of lower cross member 110, the attachment to the support columns must be made at a relatively small weld at each end. The limited size of the area available for welding is such that the connections between lower cross member 110 and columns 212 are not rigid enough to withstand a substantial moment. The lower cross members 110 therefore do not brace columns 212 in a way which might reduce their virtual length. Instead, when the supports are stacked, reliance is placed on the upper cross member 232, braced by short diagonals 230, to hold the lower end of the next upper support assembly.
The top surface of lower member 110 has a plurality of cutouts 220, shown in FIG. 6. These cutouts allow plank members 205 to be installed or removed from the scaffold system without first detaching an upper support frame that holds the planks down vertically. Planks 205 may be pushed laterally until the plank hooks 225 align vertically with the cutouts 220 of the lower cross member, whereupon the hooks drop into engagement with the upper cross member of the next lower level. However, the cutouts in the top surface of lower cross member 110 weaken its cross-section since it is no longer a closed shape and behaves like an open channel. An open channel has a reduced moment carrying capacity as compared to a closed structure.
It would be advantageous if the foregoing drawbacks of a scaffold structure with minimal cross bracing could be resolved, while maintaining its functional benefits and without introducing the drawbacks of removable diagonal bracing. What is needed is an improved support frame with a lower cross member which is more rigidly and durably connected between the support columns such that the virtual length of the support columns is reduced, while providing tapered ends to clear protruding ends of the columns for frame stacking. Furthermore the lower cross member should include the cutouts for plank removal but nevertheless provide the support of a closed shape to maximize moment carrying capacity.